1. Field of the Invention
The present disclosure relates to a sheet compression apparatus configured to compress an edge of a sheet and an image forming apparatus including the sheet compression apparatus.
2. Description of the Related Art
An image forming apparatus employing an electrophotographic method forms an electrostatic latent image based on image information formed on a photosensitive drum as an image carrier, develops the electrostatic latent image, and visualizes the image as a toner image. The visualized toner image is transferred onto a sheet using electrostatic force and the transferred toner image is fixed to the sheet by applying heat. In this manner, an image is recorded on the sheet.
A toner image which has been formed but not yet fixed to the sheet is fixed to the sheet by applying heat and pressure by a fixing apparatus. As a fixing apparatus that fixes a toner image, a heat roller type fixing apparatus is used. The heat roller type fixing apparatus fixes the image transferred onto the sheet while the sheet is nipped at a nip portion formed by a pressure roller pressing a fixing roller which internally includes a heater.
More specifically, the fixing roller which is pressed by the pressure roller has an internal heat source such as a halogen heater and its temperature is maintained at a predetermined temperature. Further, the pressure roller that presses the fixing roller has elasticity. While the sheet conveyed to the heat roller type fixing apparatus is nipped at the fixing nip portion formed by these rollers, the unfixed toner image formed on the surface of the sheet is melted. Then, the image is fixed to the sheet surface.
The base material of the fixing roller is a metal such as aluminum. Further, an elastic layer is formed on the surface of the base material using, for example, silicon rubber. Additionally, the surface of the elastic layer is coated with fluorine resin as a non-adhesive layer. The fluorine resin aids in the separation of the fixing roller from the toner. The pressure, which is applied to the fixing roller by the pressure roller to form the fixing nip portion, is generated, for example, by a spring. A great deformation in the elastic layer and the non-adhesive layer of the fixing roller is generated when the nip portion is formed due to the pressure.
In recent years, with the increase in the media types, improved durability is required with respect to image forming apparatuses, such as copying machines and printers, capable of processing various types of media. Since the fixing roller and the pressure roller are damaged by fine chipping of the surface, surface layer flaking, and rough surface layer when thick paper passes between them, although the damage may be extremely light, it becomes noticeable when the rollers are used for along time and hampers the durability of the fixing apparatus.
Under such circumstances, Japanese Patent Application Laid-Open No. 2008-298925 discusses an image forming apparatus which is capable of reducing the fine chipping of the surface and surface layer flaking of the fixing roller and the pressure roller by reducing the thickness of the sheet by applying pressure to the sheet. According to the image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2008-298925, the sheet conveyed to the fixing apparatus is made thinner by a compression apparatus arranged upstream of the fixing apparatus. Thus, the damage of the surface layer due to fine chipping and elongation of the elastic layer of the fixing roller and the pressure roller when thick paper passes the fixing apparatus can be reduced.
If the sheet is too thick, the leading edge of the sheet may not be nipped by the roller pair when the leading edge of the sheet is conveyed to the roller pair of the compression apparatus.
Although such a problem can be solved, for example, by increasing the diameter of the roller pair of the compression apparatus, if the diameter of the roller pair is increased, the whole apparatus will be larger. This is against the trend for downsizing. Further, if a metal roller pair is used, the apparatus will be heavier and handling will be more difficult.